1. Cross-Reference to a Related Patent Application
The present patent application is related to a patent application, Ser. No. 810,808 entitled "Self-Aligning Phase Conjugate Laser," by Robert W. Byren and David A. Rockwell, filed on Dec. 19, 1985, and also assigned to the Hughes Aircraft Company.
2. Field of the Invention
The present invention relates to high-energy lasers used with pointing and tracking systems. More particularly, the present invention relates to methods for aligning the reference beam of a self-aligning phase conjugate laser with an active or passive tracking sensor.
3. Background Information
Many applications of laser systems demand precise control of the direction and wavefront profile of the laser beam. A wavefront is a three-dimensional surface of constant phase, at right angles everywhere to a family of rays. Typical aberrations in the profile of the wavefront include ones that alter the phase, focus, or astigmatic characteristics of the beam. Control of these distortions and the line of sight of the beam are of paramount importance in many applications involving long-distance communications, surveying, target ranging, the guidance of weapons systems, and the delivery of laser power to a remote location.
Laser pointing and tracking systems that employ off-gimbal laser devices in conjunction with gimballed tracking sensors are susceptible to pointing errors in the line of sight resulting from (1) wander in the laser beam due to thermal refractive effects and optical bench flexure in the laser itself, (2) static flexure and dynamic motion caused by thermal stresses and vibrations, respectively, and (3) angular wander of the line of sight due to bearing runout and nonorthogonality of the gimbal axes. "Wander" in a laser beam refers to changes in position of the centroid of a laser beam profile. Bearing "runout" refers to several related phenomena which have to do with the fit of the bearing race or races to a rotating shaft; for example, radial runout refers to the radial free play of the shaft in the bearing race or races, which allows the axis of the shaft to translate a certain amount parallel to itself, or to deviate from perpendicularity in its orientation with respect to the plane of the bearing. Previous laser pointing and tracking systems have attempted to control laser beam wander through good thermal management and proper structural design, through the use of folding elements such as corner cubes and roof prisms that make the alignment insensitive to changes in their positions, and more recently through the utilization of phase conjugation of the beam.
The use of phase conjugation techniques to correct laser beam wavefront distortion is known in the art and is used in order to take advantage of the benefits that result from its incorporation in laser systems. In U.S. Pat. No. 4,005,935, for example, Wang discloses a method and apparatus for providing a phase-compensated optical beam directed at a remotely located target. The effects of phase perturbations along the path to the target are substantially cancelled, and near diffraction-limited convergence of the beam on the target is obtained.
In U.S. Pat. No. 4,321,550, Evtuhov discloses a phase conjugation apparatus that corrects for optical distortion in high-power laser systems, and minimizes optical components. His system for phase conjugate correction is particularly suitable for use with an inertial confinement nuclear fusion system.
In U.S. Pat. No. 4,233,571, Wang and Yariv disclose a laser that self-corrects for distortions introduced into the laser output beam wavefronts by aberrations and time-varying phenomena internal to the laser, such as vibrations of the cavity reflecting surfaces, warping of the reflecting surfaces through heating, misalignment of the reflecting surfaces, aberrations in the lasing medium, and turbulence in the lasing medium (if the medium is not a solid). The self-correction of the effects due to these causes allows higher system efficiency and performance of the system at its diffraction limit, i.e., at its optimum focusing capability.
Giuliano, in U.S. Pat. No. 4,429,393, discloses apparatus using phase conjugation at two different frequencies in a laser ring resonator for the purpose of providing a phase-compensated diffraction-limited output beam at either or both frequencies.
In U.S. Pat. No. 4,344,042, Hon discloses apparatus for a self-regenerative laser oscillator-amplifier that employs intracavity phase conjugation to provide compensation for optical inhomogeneities in strongly pumped laser media without suffering efficiency losses, in order to achieve single-mode output with increased average and/or peak power.
None of these inventions, however, directly addresses the problems of misalignment of the output beam of a gimballed laser system due to compliance in the gimbal structure, imperfections or wear in the gimbal bearings, and nonorthogonalty of the gimbal axes. Nor does any of them address the problems of aligning the output laser beam with a tracking sensor.
Presently the problems of structural compliance and gimbal axis wander are controlled through good mechanical design and through the use of active input beam alignment systems. Typically the input beam algnment system is a closed-loop servomechanical system that uses a collimated laser source and receiver to sense the angular deviation in pointing the beam. The closed-loop servomechanical system is utilized in combination with a precision beam-steering mirror to provide vernier correction of the disturbed line of sight. Typically such input beam alignment systems can be quite complex, are limited in servo bandwidth because of reaction torque feedback, and are themselves prone to misalignment. In U.S. Pat. No. 4,326,800, Fitts discloses such a complex system for laser beam wavefront and line-of-sight error correction. Fitts uses a low-energy reference beam at the vertex of a primary mirror that is grated to diffract a low-energy holographic replica of the high-energy primary beam. A photodetector-based servo control system compares the line of sight of the reference beam to that of the low-energy replica and generates control signals which actuate a beam steering mirror to reposition the main beam. The servo control system also includes a wavefront sensor. The sensor analyzes the wavefront profile of the low-energy replica and generates control signals which actuate a deformable mirror to correct spurious wavefront aberrations.
The use of a passive, self-aligning phase conjugation laser system provides extremely wide-bandwidth compensation of all beam wander and misalignment effects, thus overcoming one of the primary disadvantages of active input beam alignment systems. The need for strict structural design constraints on the laser and gimbal are also eliminated. The beam from a self-aligning phase conjugate laser used with a pointing and tracking system is aligned with the stabilized platform on the inner gimbal, with compensation automatically provided for angular deviations and jitter in the beam line of sight caused by gimbal motion and structural compliance. The necessity for complex electromechanical servo systems, limited in response bandwidth and themselves prone to misalignment is eliminated, and in addition, use of the self-aligning phase conjugate laser relaxes the stiffness constraints on the laser and gimbal structures.
In the case of the self-aligning phase conjugate laser, there will be additional pointing errors resulting from misalignment of the output coupling beamsplitter and any additional optics not included in the phase conjugate leg of the beam path. Also, errors in alignment of the laser oscillator and tracking sensor mounted on the stabilized platform attached to the inner gimbal are not compensated by that method.